Television receivers with remote control are becoming increasingly popular. The remote control hand unit enables the user to control the operation of the television receiver and other attachments--such as a VCR--without having to get up and touch the instrument.
In an RCA remote TV system, the remote control hand unit transmits messages to TV instrument in the form of successive bursts of an infrared (IR) carrier signal having a fixed frequency of 56.875 KHz and a period of 17.6 microseconds (.mu.s). An IR message is composed of a MARK (a burst of IR, at least 4 millisecond or ms in duration), a SPACE (an absence of IR for 4 ms), and a SYNC pulse (0.5 ms of IR) followed by 24 DATA bits. The DATA bits, in turn, consist of a 4 PREAMBLE bits indicating the subject of the message (e.g., VCR, TV, etc.), then 8 COMMAND bits specifying the actual function to be performed, followed by the logical complement of the 4 PREAMBLE and 8 COMMAND bits. A logical "zero" and a logical "one" is encoded in the spaces separating the IR SYNC pulses of constant, 0.5 ms duration. For example, a logical "zero" and a logical "one" is respectively represented by the absence of the IR carrier signal for a period of 2 ms and 1 ms between consecutive SYNC pulses. Since the complement of every data bit is also transmitted, all messages are of a fixed, 56.5 ms duration.
In the television receiver, the IR carrier signal is sensed, amplified, band-passed and decoded. The decoding of the IR carrier signal comprises of three levels of signal processing--envelope detection (i.e., detection of the IR carrier bursts), bit decoding and message decoding. The present invention is directed to a circuit for detecting the presence or absence of the IR carrier envelope.
The envelope detection circuit, in accordance with this invention, includes first and second counters, which are both clocked at four-microsecond intervals. The first and second counters are coupled to the detected IR carrier signal and count when the detected IR carrier signal is "high" and "low" respectively. The first and second counters count up to their designated upper limits (e.g., 8 and 16). Whichever counter first reaches it upper limit, it resets both the counters, and it also sets or resets, respectively, an envelope status flip flop indicating the presence or absence of the IR carrier signal. The output of the envelope status flip flop is then provided to the next stage for bit and message decoding.